This invention relates to mismatch plastic deformation. More particularly, this invention relates to a technique for inducing mismatch plastic deformation, including transformation-mismatch superplastic deformation, by chemical means.
Superplastic deformation is defined as the deformation of a workpiece to a very large strain by application of a small stress without disrupting the mechanical integrity of the workpiece. Although superplastic deformation is universally characterizable by the formula       ϵ    .    =      A    ⁢          xe2x80x83        ⁢          σ      n        ⁢          exp      ⁡              (                              -            Q                                R            ⁢                          xe2x80x83                        ⁢            T                          )            
(in which {dot over (xcex5)} is strain rate, A is a materials constant, "sgr" is stress, R is the gas constant, T is temperature and n is a stress exponent between one and two), this behavior can be produced by any of several different mechanisms. This phenomenon has been exploited in superplastic forming techniques. For example, titanium-based materials are desirable for their specific strength and stiffness at ambient and elevated temperatures but have high resistance to deformation at temperatures appropriate for traditional hot-working operations. However, titanium alloys having a fine, stable grain structure deforms superplastically, a phenomenon known as xe2x80x9cfine-grain superplasticityxe2x80x9d. Titanium-forming techniques based on fine-grain superplasticity only operate successfully within a restricted window of process parameter values. For example, only small strain rates can be imposed, so the process output rate is limited. The deformation mechanism requires that grain size be maintained within certain limits throughout the deformation process.
In another superplastic mechanism, called xe2x80x9ctransformation superplasticityxe2x80x9d (described, e.g., in U.S. Pat. No. 5,413,649, the entire disclosure of which is incorporated herein by reference), the workpiece is cycled repeatedly through a phase transformation by changing the temperature. At the end of each thermal cycle, a strain increment is produced, a phenomenon which can be termed xe2x80x9ctransformation-mismatch plasticity.xe2x80x9d The strain can be accumulated upon multiple cycles up to large overall values, at which point the material is considered to have deformed by transformation-mismatch superplasticity (or xe2x80x9ctransformation-superplasticityxe2x80x9d). It is important to note that transformation-mismatch plasticity is produced by the biasing of internal mismatch stresses and is thus distinct from xe2x80x9ctransformation-induced plasticityxe2x80x9d (TRIP) observed in many steels, in which the strain is produced by a biasing of the martensitic shape-change and not by internal mismatch stresses, and thus cannot be accumulated through multiple cycles up to large strain values.
Transformation superplasticity is advantageous compared to earlier approaches to superplasticity in that it is not limited to a workpiece material with a fine-grain microstructure and the grain growth limitation is relaxed. Also, the higher strain rates achievable result in more efficient process output. However, prolonged residence at high temperatures as required for some thermal cycling procedures can promote grain growth to sizes deleterious to the mechanical properties of the finished product. Implementing the required temperature cycling capability can be costly and difficult. Also, repeated thermal cycling can promote fatigue of the treatment apparatus.
An object of the invention is, accordingly, to provide a technique for inducing transformation-mismatch plasticity, including superplasticity, without thermal cycling .
Another object of the invention is to provide a technique for inducing transformation-mismatch plasticity, including superplasticity, that is not limited to any specific workpiece microstructure or composition and is applicable to a wide range of workpiece materials, including titanium-based materials.
Another object of the invention is to provide a powder compaction technique.
Another object of the invention is to provide a technique for forming composites.
Another object of the invention is to provide a method of inducing mismatch plasticity, including superplasticity, without causing the workpiece material to undergo a phase transition.
Another object of the invention is to provide a method of inducing transformation-mismatch plasticity, including superplasticity, that allows fast deformation of the workpiece.
Still another object of the invention is to provide a method of inducing transformation-mismatch plasticity, including superplasticity, that may be applied repeatedly to a workpiece with accumulation of deformation from each repetition.
The method of the e invention produces deformation by mismatch plasticity in a workpiece by altering the chemical composition of the workpiece materialxe2x80x94by increasing and/or decreasing the concentration of a chemical componentxe2x80x94while the workpiece is subjected to a biasing stress, in a manner that introduces a strain increment into the material, deforming the workpiece without causing, failure. Known apparatus for fine-grain superplastic forming or forging can be modified in a straightforward manner to incorporate the method of the invention by adding a mechanism for introducing and/or withdrawing a chemical component to accomplish the desired chemical composition change. The operation of the method of the invention on the workpiece is such that the mass of the workpiece is not increased thereby except in cases in which chemical component is provided to the workpiece to effect the strain increment and remains in the workpiece at the end of the operation. The method of the invention adds no bulk material to the workpiece, such as would be done by a bonding operation.
The method of the invention is not limited with respect to the deformation geometry it effects. The workpiece may be simply stretched, under uniaxial tension. Or, more elaborate shaping may be performed, for example corrugation of a sheet by superplastic forming into a die or foaming by expansion of internal cavities. The wide range of deformation arrangements achievable by the method of the invention are conveniently described with reference to the effective von Mises strain eeff, well known in the art to be defined as
eeff=⅔((e1xe2x88x92e2)2+(e1xe2x88x92e3)2+(e2xe2x88x92e3)2)xc2xdxc3xaa,
for a strain state characterizable by principal strains e1, e2 and e3. In a preferred embodiment, the deformation causes the effective von Mises strain to change by a minimum amount at some location in the workpiece. The method of the invention effects a change in the von Mises strain on the order of 0.5% or even greater. Depending on the material, the mismatch plastic strain increment may generate a change in von Mises strain of 5%, even as much as 10% and greater.
The alteration in composition under the biasing stress may be monotonic, resulting in a permanent change in the concentration of the component. Or, after completion of the deformation process the concentration change may be fully reversed, restoring the preprocess mass of the workpiece to within 0.01%, or partially reversed. In one embodiment the concentration alteration under stress is cyclic, comprising an initial change of the concentration of the chemical component in a first direction, followed by change in the opposite direction to accomplish a partial or total reversal of the initial change while the workpiece remains subject to the biasing stress. As used herein, reversing the concentration change encompasses a partial reversal as well as a full reversal, and the stipulation that the composition of the workpiece be altered while the workpiece is subject to the biasing stress denotes that a nonzero biasing stress is in effect at some time during the concentration changing operation. The biasing stress need not be constant during a unidirectional concentration change or follow the same profile during a part of the procedure adding chemical component to the workpiece as during a part of the procedure removing chemical component from the workpiece.
In one approach, the composition changes within a single-phase stability field, a concomitant change in lattice strain producing the internal mismatch and thus the strain increment without phase transformation. In another approach, the alteration in composition induces a phase transition that gives rise to an internal transformation mismatch and the concomitant strain increment. The scope of the invention is not limited with respect to type of phase transition or workpiece material, as long as it produces an internal mismatch due to volume and/or shape incompatibility between the two phases involved in the phase transition. The phase transition may be, for example, allotropic, martensitic, peritectoid or eutectoid in nature. Or the phase transition may involve precipitation of a compound due to solute saturation. The term xe2x80x9ccompoundxe2x80x9d as used herein, as it is normally understood by those in the art, does not merely mean proximal coexistence between elements, but includes specific inter-element interaction and a requires that the compound be distinct from its component elements. The method of the invention is compatible with, but not limited to, metallic ionic and covalent materials including pure metals and alloys, such as intermetallics, ceramic, polymeric or geologic workpiece materials.
The imposed change in composition in the material may affect all of the workpiece material or only a part of it. The overall deformation is generally proportional to the fraction of the workpiece involved in the alteration. As used in this document, the term xe2x80x9csegmentxe2x80x9d refers to the portion of the workpiece material undergoing a composition change and/or a phase transformation, whether it corresponds to the entire workpiece or not. The segment may, for example, form a continuous layer surrounding an unaltered core or be a collection of distinct isolated regions, each surrounded by unaltered material. In the case of phase transition, each forward or reverse transformation changes the transformed segment with respect to some aspectxe2x80x94its specific volume or, in some instances, some geometric aspect such as lattice type, lattice orientation or shapexe2x80x94so that the transformation generates an internal transformation-mismatch stress in the material. In the case of chemical composition cycling it is often desirable that the segment transformed by the reverse transformation correspond to that transformed by the previous forward transformation, so that the original phase constitution of the material is completely restored. However, the invention does not require such a correspondence; some of the material may remain in the forward-transformed state at the end of a cycle.
The transformation may occur along a macroscopic transformation front between an original phase in the material and a new phase in the material, originating in the reaction where the chemical composition change is introduced and advancing into the material in an organized fashion; or they may arise simultaneously at several discrete sites, having phase boundaries that move in random directions during transformation.
The biasing stress influences the orientation of the strain increment to produce the desired mismatch plastic deformation. The biasing stress may originate in a source either internal to, or external to, the sample; or, both internal and external sources may contribute to the bias. Residual internal stress in the workpiece may provide the biasing stress or, if the composition change induces a phase transition, the transformation stress of the phase transition may itself give rise to the bias. In a preferred embodiment, the bias is provided by an externally applied stress, the magnitude of which is chosen according to the strength of the material. Depending on the deformation desired, the externally applied biasing stress may be nonhydrostatic or hydrostatic, having a deviatoric component, such as a uniaxial or multiaxial stress. Such stresses may include tensile, compressive, torsional or bending stresses as are conventionally used to effect, for example, drawing, punching, stamping, extruding, rolling, pulling, bending, and twisting.
In a preferred embodiment, chemical composition cycling is applied to the workpiece repeatedly, each repetition introducing a strain increment. Repetitive cycling in a manner that causes the alternate induction or reversal of a phase transition to recur is especially beneficial. The change in effective von Mises strain per cycle may be as much as 1.5%, or greater. The accumulation of strain in this incremental fashion allows achievement of large overall superplastic deformations in the workpiece without applying large stresses, which would risk disruption of the mechanical integrity of the workpiece. Alternatively, a single cycle or a few cycles suffice if only a small overall strain is desired. The invention does not require that a chemical composition change applied or workpiece segment affected in any given cycle be repeated exactly in any other cycle of a repetitive series; likewise, the biasing stress exerted in one cycle need not correspond to that used in any other cycle of a series.
Although the invention is applicable to a wide range of workpiece materials, alterable by a commensurately broad range of compositional changes, mismatch plastic deformation is most efficiently accomplished if the compositional change is imposed by varying the concentration of a chemical component that can be rapidly transferred to and/or from the targeted region of the workpiece. Using a component having a high diffusivity in the workpiecexe2x80x94for example, hydrogen in titanium metalxe2x80x94or a very thin workpiecexe2x80x94such as a titanium thin film to which aluminum is addedxe2x80x94are two practical approaches. For ease of implementation, it is also desirable that the component be easily transportable to and removable from the surface of the workpiece. For example, some chemical components can be transported in the gas phase or produced by reaction at the workpiece of a species delivered in the gas phase. Such a component can then be removed by exposing the workpiece to vacuum or to another gas with zero or reduced pressure of the component, or by providing a getter to absorb the gaseous species. It is preferable that small changes in the concentration of the chemical component produce a significant strain increment.
Ordinarily the method of the invention operates on a workpiece constituted by a single cohesive body, which may be for example a large-scale aircraft-engine part or a thin film only several nanometers thick. However, the present invention may also be used for compacting an assembly of particles, initially comprising several distinct bodies (e.g., powder, wires, foils), to form a dense article. One embodiment applies the method of the invention to powder compaction, whereby a relatively dense macroscopic article is formed under pressure from an assembly of distinct very small particles, as is known in the art. For example, a typical assembly includes at least 10,000 particles each having diameter on the order of 1 to 500 micrometers or greater. The largest particle in an assembly characterized by an average particle diameter of 500 or 1000 micrometers may be as large as 2 millimeters, or even greater. The density of the assembly typically has a relative value of 65% compared to a density of 100% in the dense final product, corresponding to an average hydrostatic volume change of 35%, or a uniaxial strain of about 35÷3≈12%. Greater or smaller volume changes, such as at least 20% are also possible. For example, cold pre-compaction of the powder assembly may render an initial relative density as high as 91%, so that the chemically induced transformation-mismatch plastic deformation of the invention changes the volume by 9%.
Using the method of the invention, it is possible to obtain the transformation-mismatch plastic effects of phase transformations previously exploited by thermal cyclingxe2x80x94such as allotropic phase transformation between a lower-temperature phase and a higher-temperature phasexe2x80x94without deliberate imposition of heating or cooling operations. This approach simplifies the control equipment required to operate the treatment apparatus and decreases its energy consumption. Other benefits, related to avoiding elevated temperatures, are reduced risk of thermal fatigue of the treatment apparatus, reduced risk of undesirable grain growth in the workpiece material and reduced risk of reaction between the workpiece and apparatus.
Introduction of an alloying element that shifts the composition of the original material sufficiently so that at least some of the original material converts to a different allotropic form is one way to induce a phase transition in accordance with the invention. For example, hydrogen, vanadium and niobium are known to be beta-phase stabilizers for titanium. Adding such a stabilizer to titanium produces an alloy having a lower transus temperature between the lower-temperature alpha phase and the higher-temperature beta phase than the transus for pure titanium (about 882xc2x0 C.). Consequently, adding a sufficient amount of beta stabilizer to alpha-phase titanium causes at least some of the alpha-phase material to transform to the beta phase, with an attendant change in specific volume of the transformed material and the creation of internal transformation-mismatch stresses. Removing the beta-stabilizer from the material reverses the transformation. In a preferred embodiment, transformation-mismatch plasticity is induced in a titanium-based workpiece material by changing the concentration of hydrogen therein.
The invention is not limited to transformations accessible by the thermal pathways of the prior art but also enables transformation-mismatch plastic behavior to be induced by other transformations, not accessible through temperature change alone. The method of the invention is generally applicable to materials susceptible upon change in chemical composition to a phase transformation that generates the strain increment. Some such cycles may be executable isothermally. However, the method of the invention also encompasses process pathways that include temperature change in addition to the chemical cycling, whether the temperature change occurs simultaneously with or sequentially to the chemical change. The thermal variation may be actively imposed on the workpiece or originate within the workpiece due to the imposed change in chemical concentration.
In another preferred embodiment, hydrogen concentration is changed in a metal to cause alternate precipitation and dissolution of a second, hydride phase. This technique is especially suitable for a workpiece of a titanium-based material. Titanium hydride precipitates when hydrogen is added to titanium in excess of the hydrogen solid solubility limit. The relatively high specific volume of the hydride phase translates into a molar volume mismatch on the order of 17% with respect to the original titanium. The volume mismatch generates sufficient internal stress to produce very large transformation-mismatch plastic deformation.
In one embodiment, the original workpiece material comprises a single phase. In an alternative embodiment, the workpiece material is a multiphase composite including a matrix of one or more phases and one or more additional phases. In a preferred embodiment, the change in chemical composition of the workpiece material alternately induces and reverses a phase transition in one or more transformable phases, which may be an additional phase or part of the matrix. The composite may also include one or more phases not subject to phase transformation upon the change in chemical composition. The phase distribution is selected so as to allow forward and reverse phase transformations without interfacial decohesion. Bonding between the composite""s phases may contribute to the internal stress caused by the phase transition. With respect to the constituents of a composite material, the term xe2x80x9cphasexe2x80x9d is used herein, as it is understood by those skilled in the art, to signify a discrete, relatively homogenous part of the workpiece that is distinct from other phases in the workpiece in its physical and chemical properties.